Color stability of an electrophotographic image is inferior to that generated by offset printing, and accordingly there is market demand for improvement in this regard. In an electrophotographic image, a halftone is generally expressed by using a dither processing such as processing of screen of straight lines (i.e., line screens) or that of halftone dots. In particular, line screen processing is employed for its excellent toner attraction stability. In this type of processing, the number of line screens per unit length or area is varied with image density. However, hitherto the amount of toner attracted onto an image tends to fluctuate when the image has a high density, thereby degrading color stability. The end result is an inability to improve color density. Japanese Patent Application Publication No. 09-326927 (JP-H09-326927-A) discloses a technique of stabilizing generation of dithering dots having multiple levels and sizes while maintaining high image resolution when the image has a low area rate. However, JP-H09-326927-A simply stabilizes the dot size needed only when the image has a low area rate, and is silent as to minimizing fluctuation in the amount of toner attracted onto a high-area rate image. Consequently, a method capable of improving the color stability of high-area rate images continues to be sought.